The purpose of the cohort study reported here was to investigate the association between oral contraceptive use and risk of benign breast disease (BBD), overall and by histological subtype, within the 56,537 women in the Canadian National Breast Screening Study (NBSS) who completed self-administered lifestyle and dietary questionnaires. The NBSS is a randomized controlled trial of screening for breast cancer in women aged 40-59 at recruitment. Cases were the 2,116 women in the dietary cohort who were diagnosed with biopsy-confirmed incident BBD. For comparative purposes, a subcohort consisting of a random sample of 5,681 women (including 197 subjects with incident BBD) was selected from the full dietary cohort. After exclusions for various reasons, the analyses were based on 2,116 cases and 5,338 non-cases. There was an inverse association between use of oral contraceptives and risk of all types of BBD combined. The reduction in risk was confined largely to proliferative forms of BBD (BPED), and in particular, to those forms of BPED without histological atypia, in whom there was a progressive reduction in risk with increasing duration of use (the IRR (95% CI) for use of more than 7 years was 0.64 (0.47-0.87)); risk of BPED with atypia was increased somewhat in association with oral contraceptive use (the IRR (95% CI) for use of more than 7 years was 1.43 (0.68-3.01 )), but not in a dose-dependent manner. The results were similar when examined separately in the screened and control arms of the NBSS and for screen-detected and interval-detected BPED.
PIP: This cohort study examined the association between oral contraceptive use and the risk of benign breast disease (BBD). The Canadian National Breast Screening Study (NBSS), which employed 56,837 women aged 40-59 who completed both lifestyle and dietary questionnaires, is a randomized controlled trial of screening for breast cancer. About 2116 cases in the dietary cohort were identified and diagnosed with biopsy-confirmed BBD. The study selected 5681 women from the dietary cohort, including 197 BBD subjects for comparative purposes. Results revealed an inverse association between oral contraceptive use and risk of all types of BBD. An examination by histological subcategory revealed that risk reduction is confined largely to proliferative forms of BBD and benign proliferative epithelial disorders (BPED) without histological atypia, and with a progressive risk reduction in relation to the increasing use duration [the IRR (95% CI) for more than 7 years of use was 0.64 (0.47-0.87)]. On the other hand, the increased risk of BPED with atypia was found to be correlated with contraceptive use [the IRR (95% CI) for more than 7 years of use was 1.43 (0.68-3.01)], but not in a dose-dependent manner. The result of a separate examination in the screened and control arms of the NBSS and for screen-detected and interval-detected BPED was similar. In conclusion, this study confirms the association of prolonged contraceptive use with reduced risk of proliferative forms of BPED without atypia and increased risk of BPED with atypia.